One conventionally known type of mass spectrometer uses an ion trap for capturing (or trapping) ions by an electric field. A typical ion trap is a so-called three-dimensional quadrupole ion trap, which has a substantially-circular ring electrode and a pair of end cap electrodes placed in such a manner as to face each other across the ring electrode. In such an ion trap, conventionally, a sinusoidal radio-frequency voltage is applied to the ring electrode to form a capture electric field, and ions are oscillated and trapped by this capture electric field. In recent years, the digital ion trap (DIT) for trapping ions by applying a square wave wave voltage in place of a sinusoidal voltage has been developed (refer to Non-Patent Document 1 and other documents).
In the case where the sample is biological, a laser desorption ionization (LDI) source such as the matrix assisted laser desorption ionization (MALDI) source is often used as an ion source for generating ions to be trapped in the ion trap as previously described.
In an ion trap mass spectrometer in which the MALDI and the DIT are combined, a flash (or a pulse) of laser light is delivered to a sample, and ions generated thereby from the sample are injected into the ion trap. In this process, in order to increase the ion capture efficiency, an inert gas is introduced inside the ion trap in advance to make the injected ions collide with the inert gas to decrease the kinetic energy of the ions. This operation is called a cooling. After stably capturing the ions inside the ion trap in this manner, an ion or ions having a specific mass (or m/z, to be exact) are excited and ejected from the ion trap to be detected by a detector. A mass scan is performed by scanning the mass of the excited ions, and a mass spectrum can be created based on the detection signal obtained through the scanning.
However, in a general MALDI, one pulse of laser light irradiation often fails to generate a sufficient amount of ions, and in such a case, the signal-to-noise ratio (S/N) of the mass spectrum data obtained by one mass analysis as described above is low. Given this factor, the mass spectrum data with a high S/N is obtained by the following method. Ions are generated by a pulse of laser light irradiation; the ions are injected into the ion trap; the ions are captured and cooled; and the ions are separated with their mass and are detected. This process is repeated predetermined times (ten times for example), and the mass profiles obtained from each process are summed up on a computer.
In the above method, the more the process is repeated, the more the S/N of the mass spectrum data is improved. However, this causes a problem in that the measuring time to obtain a measurement result, i.e. a final mass spectrum, is elongated. For example, the apparatus that the inventors of the present invention used for the experiment requires a measuring time of about 1.1 seconds for one process. Therefore, about 11 seconds are required for a total of ten times, and about 33 seconds for a total of thirty times. Accordingly, the throughput of analysis decreases and the cost of analysis increases.
[Non-Patent Document 1] Furuhashi, Takeshita, Ogawa, Iwamoto, et al. “Digital Ion Trap Mass Spectrometer no Kaihatsu,” Shimadzu Review: Shimadzu Hyoron Hensyubu, Mar. 31, 2006, vol. 62, nos. 3.4, pp. 141-151.